tests: Ensure mutable deployments from libostreetest

When creating sysroots with libostreetest, we don't get the benefit of
the OSTREE_SYSROOT_DEBUG setting in libtest.sh. That means we'll get
immutable deployments that can't be easily cleaned up.

Ensure the environment variable is set before creating new sysroots. It
would be nice to set the debug flags directly, but that's private API
that isn't currently pulled into libostreetest.

Closes: #372
Approved by: cgwalters

tests/libostreetest.c

@@ -94,6 +94,9 @@ ot_test_setup_sysroot (GCancellable *cancellable,
   if (!ot_test_run_libtest ("setup_os_repository \"archive-z2\" \"syslinux\"", error))
     goto out;
 
+  /* Make sure deployments are mutable */
+  g_setenv ("OSTREE_SYSROOT_DEBUG", "mutable-deployments", TRUE);
+
   ret_sysroot = ostree_sysroot_new (sysroot_path);
 
   ret = TRUE;